## Introduction:

- The compressive strength test of concrete is required to calculate the compressive strength of the cube.
- The concrete cube of standard size is casted to determine the compressive strength and this test will performed with the help of CTM.
- Compressive strength of concrete is calculated by dividing the failure load with the area of application of load (usually performed after 28 days of curing.)
- Thus the compressive strength of concrete has unit in
*N/mm2**in SI units.* - The compressive strength of concrete is also known as the characteristic compressive strength of concrete (fck).
- In genral, the concrete strength is vary from 10
*N/mm2*to 60*N/mm2* - In Indian standards, the Compressive strength of concrete is determined by testing cube of concrete of size 150mm x 150mm.
- As per IS 516-1959 the code definition,

“The**compressive strength of concrete**is given in terms of the**characteristic compressive strength**(fck) of concrete of 150mm size cubes tested on the 28th day of curing. The**characteristic strength**is defined as the**strength**of the**concrete**below which not more than 5% of the test results are expected to fail.

Minimum or specified Compressive Strength of concrete cubes of various Grades of Concrete at 28 days of curing, are as follows.

**Compressive strength of concrete for different grades of concrete from 1 to 28 days**:

In general, concrete achieves 65-70% strength at 7 days, and this is reason for that formwork can removed at 7 days.

**Compressive strength test:**

**Apparatus:**

CTM (Compressive testing machine)

Specimen of standard size.

**Number of Specimens:**

At least three specimens should be prepared. (It can be made with different batches)

Prepare the mix design for specific concrete grade. Prepare the concrete as per mix design. And cast at least 3 specimen by using same concrete.

**Procedure:**

Casting Procedure:

- Cast at least 3 specimen by pouring concrete in mold.
- Cure all three specimen before testing. out of three, one specimen is cured for 7 days and tested on 7th day. similarly remaining specimen should be cured and tested after 14 days & 28 days respectively.

Cured specimens shall be tested immediately on removal from the water and while they are still in the wet condition.

- Place the specimen in the CTM machine
- Start the machine and apply the compressive load to specimen
- Record the maximum load applied to specimen at the first appearance of cracking.

**Calculation:**

- Size of specimen: 15cm x 15cm x 15cm
- Maximum applied load:______ tones = ____N
- Compressive strength of concrete = load applied/ area of specimen

The average value for all three specimen should be taken as the representative and it should not have difference of ±15%.

**Results of Concrete Cube Test**

Average compressive strength of the concrete cube: ………….N/ mm^{2} (at 7 days)

Average compressive strength of the concrete cube: ……… N/mm^{2} (at 28 days)